1. Field of the Invention
The present invention relates to a modular hollow-fiber-membrane filter unit for use in filtering aqueous liquid in, for example, a water purifier.
2. Description of the Prior Art
Japanese Patent Kokai Publication No. 61-171593, published Aug. 2, 1986, discloses an example of a water purifier incorporating a modular replaceable hollow-fiber-membrane filter unit, commonly referred-to in the industry as a "hollow fiber module". Water to be treated is forced to flow across an activated charcoal filter for the purpose of deodorization and is then admitted to the "module" to remove organic and inorganic impurities and particulates carried in water. The module has a housing to which a bundle of hollow fibers are sealingly bonded. Each fiber is hollow in structure and is defined by a tubular hydrophilic microporous wall acting as a membrane of filtration.
Various methods are known in the art for the manufacture of microporous hollow membrane fibers. For example, in the melt-spinning method, molten polyolefins are extruded through an annular nozzle and are drawn and stretched into a hollow fiber having a mechanically created microporous structure as shown, for example, in FIG. 1 of Japanese Patent Kokai Publication No. 58-163490, published Sep. 28, 1983. In another method, known as the wet-type phase inversion method, a solution of polysulfones and water-soluble pore-forming agent dissolved in a solvent is extruded through an annular nozzle into an aqueous deposition bath in which a hollow fiber is deposited with a microporous wall structure of the open-cell type being formed by release of the pore-forming agent.
The material forming the hollow fibers is often hydrophilic. Otherwise, it is rendered hydrophilic by subsequent surface treatment. Therefore, the micropores formed in the wall of the hollow fibers are accessed by water to be filtered which passes through the micropores to enter into the hollow fibers when a differential pressure exists across the tubular wall of respective fibers.
The problem which must be overcome in designing a modular filter unit incorporating the microporous hollow fibers is that the hydrophilic nature of the hollow fibers prevents venting of air bubbles once accumulated in the filter module. More specifically, air may inadvertently enter the module when water supply to the water purifier is momentarily interrupted for any reasons or when the water purifier is disconnected from the water supply conduit or the module accidentally pulled out of the purifier. During the normal and proper use of the purifier wherein the module is fully immersed in water, each micropore of the hollow fibers are filled with water. However, when air is once allowed to inadvertently enter the module, a film of water is retained under the surface tension in each of the micropores contacted by air bubbles because of the hydrophilic nature of the hollow fibers. Since the water film thus retained prevents air from entering through the micropores into the hollow fibers and, therefore, precludes air from being vented through the fibers, it causes air to be accumulated within the the module. The result of this is that the micropores are clogged or "locked" by air bubbles, as viewed from water to be filtered, so that the efficiency of filtration of the module is lowered.
Various efforts have been made in the prior art to prevent the "air-lock" of the module and to maintain the filtering efficiency of the hollow fiber module. For example, Japanese Patent Kokai Publication No. 58-163490, cited above, proposes use of an air vent arrangement as illustrated in FIGS. 5-7 thereof. In the embodiment thereof shown in FIGS. 5 and 7, a separate housing containing a bundle of hollow fibers is affixed to the module. In the embodiment shown in FIG. 6, a container provided with a membrane is used. Such arrangements, however, are complex in structure and are costly. Japanese Utility Model Kokoku Publication No. 63-21294, published Jun. 13, 1988, proposes use of air-pervious hydrophobic hollow fibers 2 intermingled with water-permeable hydrophilic hollow fibers 1, as shown in FIG. 1 thereof. FIG. 2 of that publication discloses hollow fibers having air-pervious portion 2 and water-permeable portion 1. Such filter module is difficult to manufacture because different fibers must be intermingled with each other. In addition, the use of air-pervious hollow fibers 2, in turn, limits the number of hollow fibers 1 effectively involved in filtration of water, so that the overall efficiency of the module is decreased.